Process fluid or gas bearings have now been utilized for some time in an ever increasing number of diverse applications. These fluid bearings, which generally comprise two relatively movable elements with a predetermined spacing therebetween, operate under dynamic conditions to form a supporting wedge of fluid therebetween sufficient to prevent contact between the two relatively movable members.
Improved fluid bearings, particularly bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively movable bearing elements. These foils, of a generally thin sheet of a compliant material, are deflected by the hydrodynamic film force between adjacent surfaces. The foils thus enhance the hydrodynamic characteristics of the fluid bearings and enable operation under more extreme load conditions than heretofore had been possible with fluid bearings.
The ready availability of relatively clean process fluid or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated bearings particularly attractive for high speed rotating machinery both as journal bearings and as thrust bearings. While in many cases the hydrodynamic or self-acting fluid bearings provide sufficient load bearing capacity solely from the pressure generated by the fluid film wedge formed by the relative rotation of the two converging surfaces, in some cases it is necessary to externally pressurize the fluid introduced between the bearing surfaces so as to further increase the load carrying capacity. These externally pressurized or hydrostatic fluid bearings do however introduce the requirement for an external source of clean fluid under pressure.
Illustrative of hydrodynamic and hydrostatic bearing patents assigned to the same assignee of this application are U.S. Pat. Nos.: 3,215,479, 3,215,480, 3,366,427, 3,375,046, 3,382,014, 3,434,761, 3,434,762, 3,467,451, 3,511,544, 3,560,064, 3,615,121, 3,635,534, 3,642,331, 3,677,612, 3,893,733, 3,951,474, and 3,957,317.
One of the problems inherent in these fluid film bearings, whether hydrostatic or hydrodynamic or whether enhanced with foils between the relatively movable bearings elements, is the initial start up of the bearing. While the introduction of the compliant foils has greatly reduced the inherent problems of start up and low speed operations, conditions still arise which make start up of such bearings difficult. The foils, being resilient, are better able to withstand the effects of frictional contacts but substantial forces can still be imposed upon them which in some cases may have the tendency to tear the foils or even yank them loose from their positioning arrangements.
One means which has been developed to overcome the inherent problems of start up and low speed operation has been the coating of the individual foil elements with a high lubricity material such as a fluorocarbon or stratified fluorocarbon. Such coatings have greatly reduced the start up loads and rubbing contacts for such bearings. These coatings are generally described in U.S. Pat. No. 3,677,612, assigned to the assignee of this application.
While the use of a high lubricity coating has further alleviated the inherent start up problems of these foil bearings, certain high speed rotating machinery applications still present difficulties particularly for thrust bearings or bearings having a thrust component. Typically, the start up of a foil bearing turbomachine is accomplished by passing high pressure air from an external source through the turbine. This flow of high pressure air will set up thrust forces on the rotating assembly which can significantly increase the frictional contact of the foil bearings. With increased static friction, higher torque is required for start up of the turbomachine which means higher gas pressure which tends to further increase the thrust forces and static friction. Because of this self-magnifying effect, it may be difficult to establish the dynamic fluid film and, in extreme cases, it may be impossible to start the turbomachine. In any case, the increased static friction from the thrust forces will accelerate bearing wear.